Discussion

Overview

So far the month of April has been warmer-than-normal in DC (+3.5°), Philly (+1.3°) and at Central Park (+1.4°) in New York City, but the overall weather pattern is about to undergo a significant change in the eastern two-thirds of the nation as we progress through the second half of April. In fact, in many ways, the upcoming weather pattern in the upper atmosphere will resemble the one that endured throughout much of the winter season. The threat of snow will even make a return to the Northeast US as we progress through the latter stages of April - at least in some of the higher elevation interior locations.

Currently, there is strong high pressure ridging across the eastern US (top, orange region) and a deep trough of low pressure in the interior western US (top, blue region). As a result, the eastern US is currently enjoying rather mild conditions whereas snow is falling in the central Rockies including in the city of Denver and several feet of the white stuff is possible just to the west of Denver during the next couple of days. All of this is about to change.

According to the latest GFS Ensemble computer model forecast, the ridge in the eastern US will be replaced by deep a low pressure trough (above, blue region) by the middle of next week and a strong high pressure ridge will form across western Canada (above, orange region). This type of 500 millibar pattern existed during much of the winter season and it promotes the transport of colder-than-normal air from Canada into the US. During this transitional period in the overall upper atmosphere pattern, there is likely to be a significant rain event in the Mid-Atlantic region at the beginning of next week and then colder air is destined to follow later in the week.

In addition to the GFS Ensemble forecast, there is corroborating evidence that colder-than-normal weather is coming to the eastern US. An index known as the North Atlantic Oscillation (NAO) is forecasted to flip in signs from positive-to-negative as we progress through the latter stages of April. The current value of the NAO index (above, in black) is clearly in positive territory, but it is forecasted to drop sharply during the next week or so (above, in red). A negative NAO index value this time of year generally supports the idea that colder–than-normal air can be transported from Canada into the central and eastern US. The latest 2-meter temperature anomaly forecast (below) by the GFS computer forecast model for the 5-day period of April 26 to May 1st shows the widespread extent of the coming colder-than-normal weather pattern.